<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        var i;
        // alert(i);
        i = 12;
        // typeof可以取变量的数据类型返回
        alert(typeof(i));
        i = "abc";
        alert(typeof(i));

        var a = 12;
        var b = "abc";
        alert(a * b); // NaN 非数字非数值
    </script>
</head>
<body>

</body>
</html>